The present invention relates to an automatic filter cutoff frequency adjusting technique, and particularly to a technique effective when applied to an automatic filter cutoff frequency adjusting circuit incorporated in, for example, a portable digital assistant.
Conventionally, a radio signal processing circuit is configured with discrete components of the respective functional blocks (an amplifier for amplifying a signal, a mixer for converting the frequency of the signal, a filter for passing only a desired band of the signal, and the like). However, recent semiconductor technology improvements have made it possible to incorporate the functional blocks configuring the radio signal processing circuit into one or more semiconductor chips. The radio signal processing circuit incorporated in the semiconductor chip converts a high-frequency signal received from an antenna into a signal of a lower frequency band with high quality (low noise, suppression of the signals of bands other than a desired signal band, and the like).
To achieve the radio signal processing circuit at low cost, it is necessary to incorporate more functional blocks configuring the radio signal processing circuit into one semiconductor chip. One of the obstacles to this objective is to incorporate a filter circuit for suppressing the signals of the undesired bands into the semiconductor chip. A SAW (Surface Acoustic Wave) filter, a dielectric filter, or the like is used as the filter circuit to suppress signals present in the bands other than the desired band. However, it is not possible to incorporate the SAW filter or the dielectric filter into the semiconductor chip.
In general, the radio signal processing circuit of discrete components is configured with a superheterodyne system which requires the SAW filter or the dielectric filter (see e.g. Non-patent Document 1 for the superheterodyne system); however, it is not possible to incorporate the SAW filter or the dielectric filter into the semiconductor chip. Accordingly, in the case where the radio signal processing circuit made of semiconductors is configured with the superheterodyne system, the SAW filter or the dielectric filter is provided outside the semiconductor chip, which increases the number of components and the mounting area.
There is proposed a radio signal processing circuit system that does not require the SAW filter or the dielectric filter, utilizing a feature of the semiconductor circuit that absolute component values vary among semiconductor chips whereas relative component values within one semiconductor chip coincide with each other with high accuracy. This system is a zero-IF system, a low-IF system, or the like, which does not require an external SAW filter or dielectric filter and suppresses signals present in the bands other than the desired band through the use of filters that can be incorporated into the semiconductor chip (it may be necessary that some filters be provided outside according to a radio system or system requirements).
In the zero-IF system and the low-IF system, a channel filter for eliminating signals other than a desired channel is disposed in a stage for processing the signal of a low frequency band after frequency conversion by a mixer circuit. By disposing the filter in the stage for processing the signal of the low frequency band, it becomes possible to implement filtering with a semiconductor circuit, namely, an active RC filter or the like in place of the SAW filter or the dielectric filter.
The channel filter suppresses signals present in a channel adjacent to the desired channel, a channel adjacent to the adjacent channel, and the like, that is, in the channels other than the desired channel. However, if a cutoff frequency which is a frequency having a gain of −3 dB from the DC gain of the channel filter shifts due to the manufacturing variation of the semiconductor, element temperatures, power supply voltage characteristics, etc., the received signal quality deteriorates.
For example, if the cutoff frequency shifts to the higher side, the degree of suppression of signals present in the adjacent channel, the channel adjacent to the adjacent channel, and the like deteriorates. If the cutoff frequency shifts to the lower side, the signal power of the desired channel decreases, so that the signal-to-noise ratio deteriorates, and the receiving sensitivity decreases. Further, in the case of receiving a digitally modulated signal, a deterioration in intersymbol interference characteristics affects the received data error rate.
Accordingly, a circuit for automatically adjusting the cutoff frequency of the channel filter is required, for example, as described in Patent Document 1. According to Patent Document 1, an automatic filter cutoff frequency adjusting circuit and a register for error correction are provided for each feedback capacitance and negative feedback capacitance (ground capacitance), thereby making it possible to adjust the filter cutoff frequency without increasing the error caused by the capacitance difference between the feedback capacitance and the negative feedback capacitance.    [Patent Document 1] Japanese Unexamined Patent Publication No. 2009-94734    [Non-patent Document 1] The Design of CMOS Radio-Frequency Integrated Circuits, CAMBRIDGE, written by Thomas H. Lee